This invention generally relates to delivery systems for probes, and is specifically concerned with a flexible, hose-like delivery system for a pancake-type eddy current coil capable of remotely and reliably transmitting rotational motion through a small-diametered conduit, such as the heat exchanger tube of a nuclear steam generator.
Systems for delivering and rotating probes for the inspection of small-diametered conduits are known in the prior art. Such systems are particularly useful in delivering and rotating pancake-type eddy current coils within the heat exchanger tubes of nuclear steam generators in order to inspect these tubes for faults such as cracks or wall thinning. However, before either the utility or the limitations of such systems can be fully appreciated, some general background as to the structure, operation and maintenance of nuclear steam generators is necessary.
Nuclear steam generators are generally comprised of a bowl-shaped primary side, a tubesheet disposed over the top of the primary side, and a cylindrically shaped secondary side which in turn is disposed over the tubesheet. Hot, radioactive water from the reactor core circulates through the primary side of the steam generator, while non-radioactive water is introduced into the secondary side. The tubesheet hydraulically isolates but thermally connects the primary side to the secondary side by means of a number of U-shaped heat exchanger tubes whose bottom ends are mounted in the tubesheet. Hot, radioactive water from the primary side flows through the interior of these heat exchanger tubes while the exterior of these tubes comes into contact with the non-radioactive water in the secondary side in order to generate non-radioactive steam.
In the secondary side of such steam generators, the legs of the U-shaped heat exchanger tubes extend through bores present in a plurality of horizontally-oriented support plates that are vertically spaced from one another, while the ends of these tubes are mounted within bores located in the tubesheet. Small, annular spaces are present between these heat exchanger tubes and the bores in the support plates and the tubesheet which are known in the art as "crevice regions". Such crevice regions provide only a very limited flowpath for the feedwater that circulates throughout the secondary side of the steam generator, which causes "dry boiling" to occur wherein the feedwater boils so rapidly that these regions can actually dry-out during the operation of the generator. This chronic drying-out causes impurities in the water to precipitate and collect in these crevice regions. These precipitates ultimately creates sludge and other debris that promote the occurrence of corrosion in the crevice regions which, if not repaired, can ultimately cause the tube to crack and to allow radioactive water from the primary side to contaminate the non-radioactive water in the secondary side of the generator.
Eddy current probe systems were developed to monitor the extent to which the walls of such heat exchanger tubes were degraded as a result of corrosion. One of the latest generations of such probes are known as "pancake-type" eddy current probes. Such probes comprise a cylindrical body that is insertable within the interior of the tube to be inspected, and a small, relatively flat circular coil mounted on the side of the probe body. The coil is resiliently biased radially into wiping engagement with the inner wall of the heat exchanger tube. In operation, a miniaturized motor (operating through a gear train), and lead screw simultaneously rotate and linearly advance the probe body so that the small flat pancake coil that is resiliently mounted on the side of the probe body scans the interior wall of the heat exchanger tubes along a helical path.
While such prior art eddy current coil systems have proven themselves to be an effective tool in accurately and reliably inspecting the inner walls of heat exchanger tubes for cracks, pits, wall thinning and other degradations which are caused by corrosion, the applicant has observed a number of areas where such systems could stand improvement. For example, the miniaturized motors, drive trains and electric slip rings contained within the bodies of such probes to create the necessary helical movement of the pancake coil are expensive, and require a considerable amount of effort to assemble within the narrow confines of the probe body, whose diameter can only be about 0.50 inches in a probe system capable of inspecting heat exchanger tubes having a 0.625 inch outer diameter. These expenses are compounded by the fact that the probe bodies and all related delivery conduits are typically discarded after a single maintenance operation in a nuclear power plant due to radiation contamination. But even if they were not so discarded, the applicant has noticed that the electrical load placed upon the relatively delicate windings of the miniaturized motors used to create the required helical motion can prematurely jeopardize the reliable operation of these motors, and can ultimately cause them to burn out well before their expected lifetimes.
One possible solution to some of the problems associated with the prior art would be the development of more powerful and reliable miniature motors. However, further developments in such motors would be a relatively costly and time consuming endeavor, and still would not solve the cost problems associated with the fact that the probe bodies and all associated delivery conduits are typically discarded after a single maintenance operation. Another possible solution might be to remotely drive the probe body by means of a flexible power shaft that is mechanically coupled to a motor located well outside of the heat exchanger tube being inspected. However, known flexible power shafts are not compatible with either the pusher-puller mechanisms used to extend and withdraw the probe body into and out of the radioactive primary side of the generator, or with the robotic arms typically used to insert and withdraw this probe body from a selected heat exchanger tube within the primary side. Such puller-pusher mechanisms employ opposing, motor-driven rollers which are resiliently mounted to engage and drive a cable that is connected to the proximal end of the probe body in order to move the probe into and out of the primary side, while robotic positioners use reciprocating gripper mechanisms for extending and retracting first the probe body and then the cable attached thereto into and out of a heat exchanger tube. The resilient rollers and grippers used in these mechanisms would mechanically interfere with the transmission of torque if the probe body were directly connected to a flexible, rotating shaft. Of course, a flexible, plastic conduit might be placed over the rotating shaft in order to prevent such interference from occurring. However, experience has shown that the friction that develops between the flexible, rotating shaft and the inner walls of any such covering greatly interferes with the smooth transmission of torque over the distances required to remotely rotate a probe disposed within a heat exchanger tube in a nuclear steam generator. Moreover, such known flexible shafts are insufficiently flexible to be wrapped around the reels used in conjunction with known pusher-puller mechanisms.
Clearly, what is needed is a flexible delivery system for positioning and rotatably driving a probe at a desired position within a conduit such as a heat exchanger tube which obviates the need for expensive miniaturized motors, drive trains and slip rings. It would be desirable if such a system were relatively inexpensive so that it could be discarded after becoming radioactively contaminated incident to a single inspection and maintenance operation without major cost. Finally, it would be desirable if such a system were compatible with known pusher-puller mechanisms and robotic positioning devices, and were simple in construction, and reliable and accurate in operation.